1. Field of the Invention
The present invention relates to linear power amplifiers. More specifically, the invention relates to distortion cancellation in power amplifiers.
2. Description of the Related Art
Power amplifiers are critical components for all modern communication systems because they provide the power that enables the communication signal to propagate over the desired distances of the communication system. Communication systems are allocated defined portions of the frequency spectrum and these allocated portions, or bands, are a limited resource. Therefore, there is a strong economic incentive to use these bands as efficiently as possible by maximizing the amount of data transmitted per frequency range. Modulation techniques, such as quadrature amplitude modulation (QAM), or multi-carrier methods, such as orthogonal frequency division multiplex (OFDM), used for high data rate signals are very sensitive to signal distortion and require linear power amplifiers that do not distort the signal during the amplification process.
FIG. 1 is a schematic diagram illustrating the feed forward technique of distortion cancellation. In FIG. 1, an input signal, A, is split by splitter 105 into two portions, A1 and A2, that are directed into two paths. The lower path includes a delay line 110, a summer 130 and an error amplifier 150. The upper path includes a power amplifier 120, a coupler 140 and a delay line 160. An attenuator 145 supplies a portion of the signal from the upper path to summer 130 in the lower path; and the signals in the two paths are recombined by summer 165. In the upper path in FIG. 1, the signal exiting power amplifier 120 contains both an amplified portion of the input signal, K1A1, and distortion D generated by the nonlinearities of the power amplifier. A portion, αK1A1+αD, of the amplified signal is directed by directional coupler 140 along the upper path into delay line 160, which matches the delay caused by the error amplifier 150. The remaining portion, βK1A1+βD, of the amplified signal is directed by the directional coupler 140 into an attenuator 145. The attenuator 145 attenuates the amplified signal such that the its amplitude at the output of the attenuator matches that of signal A2. The input signal A2 diverted into the lower path of FIG. 1 is directed into a delay line 110. Delay line 110 is adjusted to compensate for the delay caused by the power amplifier 120, coupler 140, and attenuator 145 such that the output of the delay line arrives at the summer 130 at the same time as the attenuated signal from the attenuator. The delayed input signal is subtracted from the attenuated signal at the summer 130 such that the output signal of summer 130 is only an attenuated portion of the distortion signal. The distortion signal is amplified by the error amplifier 150 such that the amplitude and phase of the output of amplifier 150 matches the amplitude and phase of the distortion signal component of the signal exiting delay line 160. Summer 165 subtracts the amplified distortion signal from error amplifier 150 from the signal from delay line 160 leaving an output signal 111 that contains little or no distortion.
The feed forward design is susceptible to temperature variations and other factors and the delay lines must be carefully matched for wideband signals. Therefore, there remains a need for improving the linearity of power amplifiers by canceling the distortion caused by power amplifier nonlinearities.